Manufacture of alternating current rectifiers of the selenium type



Patented Nov. 13, 1945 OFFICE MANUFACTURE OF ALTERNATING CUB- BENT RECTIFIERS OFIHE SELENIUM TYPE Leslie Ernest Thompson and Alexander Jenkins,

London N. 1, England, assignors, by mesne assignments, to 'l'he Union Switch and Signal Company, Swissvale, Pa a corporation of Pennsylvania No Drawing. Application January s, 1943, Se-

ll'iigalzNo. 471,737. In Great Britain January 21,

'1 Claim. (c1. 175-360) This invention relates to the manufacture oi alternating current rectifiers of the selenium type comprising a base or support provided with a layer or coating containing selenium and other substances, a counterelectrode being applied to the surface of'this layer.

The invention more particularly relates to the manufacture of rectiflers of this character in which the layer or coating contains in addition to the selenium, selenium dioxide and a halogen salt such for example as sodium chloride, one process of manufacture of material of this kind being described in United States Patent No. 2,307,474.

Important electric characteristics of a rectifier of this character are the resistance in the iorward direction, the resistance in the reverse direction and the so-called "bottom bend" characteristic, that is to say, the applied voltage in the forward direction at which eiiective rectification takes place, the efficiency of the rectifier in commercial service being dependent upon these characteristics, all of which vary within considerable limits in accordance with the particular composition of the selenium mixture employed, the eventual resistance oi. the rectifier in the reverse di-. rection being also determined by an electric forming process described for example in U. 5. Patent No. 2,279,187. I

It is found in practice that the resistance in the forward direction is favourably aflected by the introduction of relatively small quantities of certain other substances into the selenium mixture and according to the present invention a'relatively small amount of sulphur either alone or in combination with other material is added to the a mixture for the purpose of reducing the forward resistance of the rectifier.

The amount of added sulphur for this purpose is dependent upon the purity of the commercial selenium employed and is usually from 0.04% to 0.08%.

In a typical example the selenium composition prepared as described in United States Patent No. 2,307,474 the addition of 0.04% of sulphur is found to reduce the forward resistance by approximately 30% while the addition of 0.04% of sulphur and 0.02% of iron causes a reduction in the iorward resistance of approximately-22%. The improved process of the invention is not however limited to the values given above by way of example which may be varied in accordance with the composition of the selenium mixture employed. The improved process of the invention is likewise not limited to a selenium composition prepared as described in Ignited States Patent No. 2,307,474, but can be used in the preparation of any alternating current rectifier oi the kind comprising a base or support provided with a layer or coating containing selenium.

Having now described our invention, what we claim as new and desire to secure by Letters Patent is:

An alternating current rectifier oi the kind described comprising a base or support provided with a layer in the form oi a selenium mixture containing selenium, selenium dioxide, a halogen salt approximately 0.04% of sulphur and approximately 0.02% of iron.

LESLIE ERNEST THOMPSON. ALEXANDER JENKINS. 

